1. Field of the Invention
This invention in general relates to the field of injection molding and in particular to adjustable sprue bushings for conveying melted plastic from the nozzle to a mold cavity gate or runner system.
2. Description of the Prior Art
In plastic injection molding processes, a thermoplastic or thermoset molding compound is first heated to plasticity in an injection cylinder at controlled temperature. Afterwards, the plasticized compound is forced from the cylinder through a nozzle by means of pressure generated within the cylinder. On emerging from the nozzle, the plasticized compound is conveyed through a hole in a mold plate, usually stationary, by means of a sprue bushing (sprue for short) into the runner system or gate of the mold cavity. Once in the cavity, the resin, assumes the shape of the cavity,is cooled to the point where it is sufficiently solidified to retain the desired cavity shape. The mold is then opened, and the part ejected or otherwise removed. The entire process is usually automated with the clamping of the mold parts prior to injection and unclamping for part ejection or removal after cooling taking place under the control of a microprocessor or other form of automated controller. Machines are rated by the number of ounces they will inject per piston or screw stroke and by the square inches of working area that can be clamped against injection pressure. Consequently, a machine can be used to fabricate a variety of different sized parts up to its maximum capacity. This is accomplished by using, among other things, plates of suitable thickness for the part at hand; large working areas and high injection pressures calling for robust mold plates to mount and sustain the large stresses created during the molding cycle. Plates used to make small parts, on the other hand, can be scaled down in size in accordance with the reduced stresses generated while molding them. Parts between the largest and smallest obviously require intermediate sized plates, and the industry has adopted standard plate thicknesses to promote productivity.
For best process performance regardless of part size, it is known to be beneficial to control the temperature of the plasticized compound more or less constant throughout its travel to the cavity. This reduces process problems associated with material degradation due to thermal variability, improves yield by decreasing scrap losses, and increases machine production time by reducing down time due to freeze offs.
However, while standardized in many respects, particularly with respect to mold base or plate thicknesses, present molding machinery does not always provide for precise temperature control to take advantage of its beneficial effects. Indeed, much of the available machinery is still run employing cold sprue bushings which allow the temperature of the resin to be poorly controlled from the time it leaves the nozzle until it reaches a zone in the mold where temperature control is reacquired with, for example, internal heating channels in the mold.
More sophisticated practice, apparently not yet universally accepted, recognizes the advantages of controlling temperature by employing hot sprue bushings to convey material from the nozzle to the cavity gate often times through the fixed mold plate, sometimes referred to as the "A" plate or base. A variety of approaches for providing heat in these hot sprue bushings have been used. Among these are the use of resistive heating elements and heat pipes such as those described in U.S. Pat. No. 4,034,952 entitled "HOT PLASTIC INJECTION BUSHING" issued on Jul. 12, 1977. In the latter case, the heat pipes are used to transfer heat from electrically powered heater bands located at the nozzle end of the sprue bushing to regions along the bore near the tip.
The known commercially available hot sprue bushings, however, have a number of disadvantages in that molding shops need to acquire a variety of different sizes to match the range of standard "A" plate dimensions adopted by the industry for use in fabricating parts of different scale and it is expensive and time consuming to customize them. In addition, their reliability is impaired because of failures associated with tip breakage and heating performance.
Consequently, it is a primary object of this invention to provide a highly reliable hot sprue bushing that can easily be adapted for use with the full range of molding plate thicknesses or easily cutomized to meet the needs of specific jobs where there is not an exact match between off-the-shelf bushings and standard plate thicknesses.
It is another object of this invention to provide a hot sprue bushing for uniformly controlling the temperature of plasticized material while it travels from the nozzle to the mold cavity.
Other objects of the invention will in part be obvious and will in part appear hereinafter. A full understanding of the invention will best be had from reading the detailed description to follow in connection with the detailed drawings.